In this study, the mechanical properties of nano-silica particulate-reinforced epoxy composites with different crosslinking densities were clarified experimentally to consider the interaction effects between nano-particles and the network structure in matrix resin. The matrices were prepared by curing with an excessive mixture of diglycidyl ether of bisphenol A type epoxy resin as the curing agent for the stoichiometric condition. The volume fraction of the silica particles with a median diameter of 240 nm was constantly 0.2 for every composite. The crosslinking densities and glass transition temperatures of the neat epoxy resins were identified from thermo-viscoelastic properties measured by dynamic mechanical analysis. Elastic moduli and strengths of the composites and the neat epoxy resins were measured by three-point bending tests. The glass transition temperatures of the neat epoxy resins decreased linearly as the crosslinking densities decreased from the stoichiometric condition. The glass transition temperatures of the composites were reduced by adding the nano-silica particles. The bending moduli of the composites in the glassy state could be predicted by using a mixture law of the composites regardless of the crosslinking densities and glass transition temperatures. The bending strengths were found to be sensitive to the crosslinking densities: they were both higher (for composites with high crosslinking densities) and lower (for composites with low crosslinking densities) than those of the neat epoxy resin. These results demonstrate that the interaction between nano-particles and network structures reduces the bending strengths, especially for low crosslinking densities.